title: Polyethylene glycol enhances the binding of C1q to circulating immune complexes.
abstract: By radioimmunoassay we measured the amount of endogenous C1q that was precipitated by polyethylene glycol (PEG) under the conditions of the 125I-C1q-binding test (C1q-BT). We found a linear correlation between the percentage endogenous C1q that was precipitated and the 125I-C1q-binding activity (C1q-BA). We concluded that the 125I-C1q behaves like the endogenous C1q. To detect circulating immune complexes (CIC) which had already bound C1q, human sera were added to tubes coated with anti-C1q. Under the conditions used, no C1q-bearing CIC were detected. In addition, 7 sera from patients with high C1q-BA were analyzed by sucrose-gradient ultracentrifugation. No C1q was found in the fast sedimenting fractions, although C1q-BA was detected in these fractions. With IgG-coated tubes we observed that PEG enhanced the binding of 125I-C1q as well as endogenous C1q to aggregated and monomeric IgG. PEG also enhanced the binding of CIC to C1q-coated tubes. The results suggest that CIC detected by the C1q-BT do not bear C1q in significant amounts in the circulation and that these CIC become detectable only in the presence of PEG.
mesh_terms: D000818:Animals; D000936:Antigen-Antibody Complex; D003166:Complement Activating Enzymes / Q000378:metabolism*; D015922:Complement C1q; D006801:Humans; D005779:Immunodiffusion; D007074:Immunoglobulin G; D011092:Polyethylene Glycols / Q000494:pharmacology*; D011817:Rabbits; D011863:Radioimmunoassay
chemical_list: D000936:Antigen-Antibody Complex; D007074:Immunoglobulin G; D011092:Polyethylene Glycols; D015922:Complement C1q; D003166:Complement Activating Enzymes